Glass-ceramic articles

ABSTRACT

THIS INVENTION RELATES TO THE MANUFACTURE OF GLASS-CERAMIC ARTICLES IN THE PBO-AL2O3-SIO2 COMPOSITION FIELD UTILIZING TIO2 AND/OR ZRO2 PLUS P2O5 AS NUCLEATING AGENTS. ALPHA-CRISTOBALITE COMPRISES THE PREDOMINANT CRYSTAL PHASE WHEN THE SILICA CONTENT OF THE ARTICLES IS AT A HIGH LEVEL WHEREAS LEAD FEDSPAR CONSTITUTES THE PREDOMINANT CRYSTAL PHASE WHEN THE LEAD OXIDE CONTENT OF THE ARTICLE IS AT A HIGH LEVEL. AT INTERMEDIATE CONCENTRATIONS OF PBO AND SIO2, THE FINAL CRYSTALLIZATION WILL BE A MIXTURE OF THESE TWO PHASES.

United States Patent 3,700,471 GLASS-CERAMIC ARTICLES David A. Duke, Corning, N.Y., assignor to Corning Glass Works, Corning, N.Y. No Drawing. Filed Nov. 12, 1970, Ser. No. 89,111 Int. Cl. C03c 3/22, 3/10; C04c 35/00 US. Cl. 106-39 DV 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to the manufacture of glass-ceramic articles in the PbO-Al O -SiO composition field utilizing TiO and/or ZrO plus P 0 as nucleating agents. Alpha-cristobalite comprises the predominant crystal phase when the silica content of the articles is at a high level whereas lead feldspar constitutw the predominant crystal phase when the lead oxide content of the article is at a high level. At intermediate concentrations of PhD and SiO the final crystallization will be a mixture of these two phases.

Glass-ceramic articles are produced through the controlled crystallization of glass articles. An extensive discussion of the manufacturing techniques, the structure of the articles, and the theoretical considerations involved in the production of glass-ceramic articles is set out in U.S. Pat. No. 2,920,971 and reference is hereby made to that patent. In general terms, however, the manufacture of glass-ceramic articles contemplates three principal steps: (1) a glass-forming batch to which a nucleating agent is commonly added is melted; (2) the melt cooled sufficiently rapidly to form a glass article; and (3) the glass article is subjected to a defined heat treatment such that relatively uniformly-sized, fine-grained crystals homogeneously dispersed in a glassy matrix are crystallized in situ. The amount of crystallization is such that the glass-ceramic article is predominantly crystalline, i.e., greater than 50% by weight crystalline. Ordinarily, the heat treatment comprises two stages: (1) the glass article is heated at a temperature above the transformation range of the glass but below the softening point thereof to cause the development of nuclei therein; and (2) the nucleated article is heated to a temperature above the softening point of the glass to expedite the growth of crystals on the nuclei.

Inasmuch as a glass-ceramic article is produced through the crystallization in situ of a glass article, the final product is free from voids and non-porous. And, since a glassceramic article is more crystalline than glass, the physical and chemical properties thereof will be more characteristic of the crystal phase than of the parent glass. Finally, as can be appreciated, the residual glassy matrix will have a far different composition from that of the original glass article since the crystal components will have been precipitated out.

I have discovered that certain glasses of high silica content in the PbO-Al O -$iO composition field, when nucleated with TiO and/or a combination of ZrO +P O can be crystallized in situ to yield a glassceramic article having coefficients of thermal expansion ranging between about ISO-200x per C., and having the capability of exhibiting very high strength when coated with a low expansion glaze. I have also discovered that certain glasses in the same general composition field but having a higher PbO content can be crystallized in situ to glass-ceramic articles exhibiting coefiicients of thermal expansion between about 40-70 10""/ C. thereby rendering them especially suitable for coating and bonding with such refractory metals as tantalum, tungsten, molybdenum, and alloys thereof.

In broad outline, my invention comprises melting a batch for a glass consisting essentially, by weight on the oxide basis, of about 10-45% PbO, l025% A1 0,, 35-- SiO and a nucleating agent selected from the group consisting of 6-l2% TiO and/or 3-10% ZrO +5-15% P 0 said nucleating agent not exceeding about 23% of the total composition. The melt is simultaneously cooled to at least below the transformation range thereof and a glass body shaped therefrom. The glass body is thereafter heated to a temperature between about 700-1200 C. for a period of time suflicient to attain the desired crystallization in situ. The transformation range is that temperature at which a liquid melt is considered to have been converted into an amorphous solid, this temperature normally being deemed to lie between the strain point and the annealing point of a glass. My preferred heat treatment practice contemplates a two-step procedure wherein the glass article is initially heated to temperatures somewhat above the transformation range, i.e., about 700-850 C., and held thereat for a sufficient length of time to assure satisfactory nucleation and to begin crystal growth. Thereafter, the article is heated to about 900-1200 C. and maintained thereat for a sufficient period of time to complete crystal growth. Inasmuch as the crystallization step is a time-temperature dependent process, it can readily be understood that at temperatures within the higher extreme of the crystallization range only brief dwell periods will be required, e.g., about hour or even less; whereas, at the cooler end of the crystallization range, maintenance times as long as 24-28 hours may be necessary to achieve extensive crystallization.

It will be appreciated that numerous modifications in the manufacturing process are possible. For example, when the melt is quenched and shaped to a glass article, the article may be cooled to room temperature for visual inspection of the glass quality prior to initiating the heat treating step. Nevertheless, where speed in production and fuel economies are sought, the melt may merely be quenched to a glass shape at a temperature just below the transformation range and the heat treatment thereof begun immediately. Also, although a two-step heat treatment cycle is preferred, a very satisfactory product can be obtained when the glass article is simply heated from room temperature or the transformation range to temperature within 700-l200 C. range and held thereat for a sufiicient length of time to develop the extensive crystallization desired. Further, if therate of heating is relatively slow and the final crystallization temperature is near the upper extreme of the heat treating range, no dwell period, as such, at any one temperature will be required. However, since the growth of crystals is dependent upon time and temperature, the rate of heating the glass article above the transformation range must not be so rapid as to prevent the growth of suflicient crystals to support the article, thereby leading to consequent deformation and slumping of the article. Hence, whereas heating rates of 10 C. per minute and higher have been employed successfully, particularly where physical supports were provided for the glass articles, I prefer a rate of about 3 5 C./minute. These heating rates have yielded articles exhibiting very little, if any deformation throughout the group of compositions comprising this invention.

In my preferred two-step heat treatment procedure, a dwell time of about 1-6 hours within the nucleation range followed by a crystal growth period of about 1-8 hours within the crystallization temperature range is normally utilized.

Table I records compositions, expressed in weight percent on the oxide basis, of thermally crystallizable glasses which were heat treated to produce uniformly fine-grained glass-ceramic articles. The batch ingredients may comprise any materials, either oxides or other compounds which, on being melted together, are transformed to the sirably present in amounts totalling less than by desired oxide compositions in the proper proportions. The weight. The inclusion of Li O to the high silica composibatch ingredients were compounded, ballmilled together tions leads to the growth of low expansion beta-eucryptite to aid in obtaining a homogeneous melt, and thereafter or beta-spodumene crystals which are not claimed as part melted in open platinum crucibles for about 16 hours at 5 of this invention. Therefore, Li O is preferably absent temperatures between about 15001600 C. Glass cane from the compositions and can only be tolerated in samples of about A" diameter were drawn from each amounts less than about 2% by weight. The inclusion of melt and the remainder poured onto a steel plate to give ZnO and MgO can result in the crystallization of gahnite a circular patty about 5" in diameter and A" thick. The (ZnO-Al O and spinel (MgO-Al O respectively and glass articles were immediately transferred to an annealer amounts up to about 10% by weight total of these oxides operating at 600 C. Following the annealing cycle, the can be employed. The development of the feldspar phase glass articles were placed in an electrically-fired furnace is substantially benefited through the presence of BaO. and exposed to the heat treatment schedules reported in Therefore, amounts up to may be added, partic- Table II. At the conclusion of the heat treating step, the ularly in those glass compositions where the lead oxide current to the furnace was cut off and the crystallized 15 concentration is at a low level and the feldspar phase is articles were either taken out directly into the ambient desired. atmosphere or merely left in the furnace and permitted In general, I have discovered that where the PhD conto cool to room temperature within the furnace. The tent of the glass composition is below about 25% by furnace rate of cooling was estimated t average b t weight, alpha-cristobalite will constitute the vast bulk of 3-5 C./minute. the crystallization unless some mineralizer such as BaO is Although the above-recited quantities of P1 0, A1 0 included in the composition to promote the crystallization SiO and nucleating agent are demanded to secure a glassof a feldspar stfllcture- In glasses Whfire Pbo comprises ceramic article wherein the crystal phase consists essenmore than about y Weight, With Without tially of alpha-cristobalite and/0r le d f ld mineralizer, the crystallization of the product will consist essentially of the lead luminosilicate phase.

(Pbo A1203 68102) 25 The melts resulting from the glasses of Table I are quite and exhibiting a coefiicient of thermal expansion ranging fiuid and no fining agent was required. However, a conbetween about 40-200 10"/ C., depending upon the ventional fining agent such as As O may be added as relative proportions of the two primary crystal phases in desired.

TABLE I.PERCENTAGES the crystallized product, minor amounts of compatible Table II records the heat treatment schedule to which metal oxides up to a total of about 15% by weight may each example was subjected, a visual description of each be included to aid in melting the batch, forming the melt, crystallized article, a measurement of the coefiicient of or modifying the chemical and physical properties of the thermal expansion (25 "-300 C.), and the crystal phases final article. Such additions as B 0 Na O, K 0, CaO, present as determined by X-ray diffraction analysis.

and SrO appear to inhibit the crystal growth so are de- TABLE II C 11. Example No. Heat treatment Description Crystal phases xi 8- 1 Heat to 850 C. at 300 C.Ihr.; hold for 2 hours; heat to White, finely crystalline Cristobalite, rutile 1150 Ct. at 300 G.lhr.; hold for 2 hours; cool at turnaee ra e.

2 Heat to 850 C. at 300 C./hr.; hold for 2 hours; heat to .-;-;..do Lead feldspar 6 1150 Ct. at 300 C./hr.; hold for 3 hours; cool at furnace ra e.

3 Heat to 800 C. at 3 C.lhr.; hold for 2 hours; heat to '."..-.d0 Cristobalite, gahnlte, rutile 202 1100 Ct. at 300 C./hr.; hold for 4 hours; cool at furnace ra e.

4 Heat to 850 C. at 300 C./hr.; hold for 2 hours; heat to White, very finely crystalline- Cristoballte, cubic ZrO:

1100" C; at 300 C./hr.; hold for 2 hours; cool at iurnace ra e.

5 Heat to 850 C. at 300 C./hr.; hold for 4 hours; heat to -....do Cristobalit cubic ZrO; 146

1050 Ct. at O./hr.; hold for 4 hours; cool at furbaddeleyite. nace ra e.

e Heat to 850 0. 300 C.Ihr.; hold for 4 hours; heat to White, finely crystalline Cristoballte 1100 C. at 00 C./hr.; hold for 4 hours; cool at furnace rate.

7 Heat to 850 C. at 30 C./hr.; hold for 4 hours; heat to .....do

1100 0.1: at 100 C./hr., hold for 4 hours; cool at turnace ra e.

8 Heat to 800 C. at 300 C./hr. hold for 4 hours; heat to White, very finely erystalline. Cristobalite 1050" Ct. at 100 O./hr.; hold for 2 hours; cool at (urnace Ia e.

TABLE II'\Continued Coefiz'. exp. Example No. Heat trealtment Description Crystal phases C.)

9 Heat to 850 C. at 300 0. /hr.; hold for 2 hours; heat to White, finely crystalline 1100 C. at 300 O./hr.; hold for 2 hours; cool at furnace rate.

10 Heat to 800 C. at 300 C./hr.; hold for 2 hours; heat to do 1100 C. at 100 C./hr.; hold for 2 hours; cool at turnace rate.

11 Heat to 850 0. at 300 C.Ihr.; hold for 2 hours; heat to Gold-white, finely crystalllne. Barium-lead feldspar 43 1150 C. at 300 G./hr.; hold tor 2 hours; 0001 at lurnace rate.

12 Heat to 850 C. at 300 O./hr.; hold for 2 hours; heat to White, finely crystalline 1060 C. at 300 C./hr.; hold for 2 hours; 0001 at iurneee rate.

13 Heat to. 800 C. at 300 C.lhr.; hold for 2 hours; heat to do 1000 C. at 100 O./hr.; hold for 2 hours; 0001 at furnace rate.

Tables I and II amply demonstrate the composition and Percent process parameters for producing glass-ceramic articles Na O 3.0 exhibiting coefiicents of thermal expansion ranging about K 0 1.1 40-200 X 10'' per C. and containing alpha-cristobalite ZrO 0.8 and/or lead feldspar as the principal crystal phase. Where CdO 0.3 the crystal phase consists essentially of alpha-cristobalite, CaF 1.1

the coeflicient of thermal expansion of the glass-ceramic articles ranges about 150-200x10- C. whereas, when the crystal phase consists essentially of lead feldspar, the coeflicient of thermal expansion of the articles ranges about -70 10-"/ C. The crystal content of the arti- 30 cles is greater than about by weight and normally exceeds about by weight, depending upon the extent to which the components of the batch are adaptable to the formation of crystal phases. The crystals, themselves,

are reasonably uniformly fine-grained, substantially all 35 Percent Si0 47.8 PbO 21.0 2 3 9.7 50 CaO 7.9 Al O 7.3

I claim:

1. A glass-ceramic article exhibiting a coeflicient of thermal expansion (25-300 C.) between about 200 10" and consisting essentially of fine-grained alpha-cristobalite crystals substantially uniformly dispersed in a glassy matrix, said crystals comprising the major proportion of the article and being formed through crystallization in situ from a glass body consisting essentially, by weight on the oxide basis, of about 1025% PbO, 10-25% A1 0 35-65% S102, and a nucleating agent selected from the group consisting of 6-12% TiO and/or 33-10% ZrO +515% P 0 the sum of TiO-,;,, ZrO- and P 0 constituting no more than about 23% of the total composition.

References Cited UNITED STATES PATENTS 2,691,855 10/ 1954 Armistead 10639 DV 2,920,971 1/ 1960 Stookey 106-39 DV 3,586,522 6/ 1971 'Hofi'man 106-39 DV JAMES E. POER, Primary Examiner M. L. BELL, Assistant Examiner US. Cl. X.R. 106-53 UN TED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,7oo, r71 DATED October 2 1972 |NVENTOR(S) David A. Duke It is certified that error appears in the ab0ve-identified patent and that said Letters Patent are hereby corrected as shown below:

Table II, Example 2, Heading Coeff. exp. (x lO /C.), "6" should be 6h gigned and Sealed this A rresr:

RUTH C. MASON 4 QMARSHALL DANN Arresting Officer ummissr'mwr nj'lare'nls and Trademarks 

